An Innovation in Ketene Chemistry

烯酮反应中1，3-二烯的不识别一直是有机化学中的一个重要问题。我们在这项研究中解决了这个问题。从理论和实验两方面研究了烯酮-二烯环加成反应的机理。通过精确的实验和从头计算追踪了二苯基乙烯酮与环状和线性二烯的环加成反应的路径。所得结果表明，加成路径和最终产物取决于二烯的结构，环状（具有s-顺式）或线性（具有s-顺式和s-反式）。二苯基乙烯酮与环二烯的环加成反应是两步反应，包括[4 + 2]加成和Clasien重排，得到[2 + 2]环加成物。二苯基乙烯酮与环（顺）1，3-二烯的反应是由[2 + 2]加成和逆Claisen重排[4 + 2]环加成反应组成的两步反应，反应产物经[2 + 2]加成和逆Claisen重排[4 + 2]环加成反应得到了较好的环加成产物。 ...更多信息 首次用低温核磁共振波谱法检测了Diels-Alder环加合物（α-亚甲基二氢吡喃）。最初形成的环加合物通过[3，3] σ转移（Claisen）重排被最终的[2 + 2]型产物环丁酮转化。相反，烯酮与开链1，3-二烯（具有s-顺式和s-反式形式）反应，最初形成[2 + 2]型和[4 + 2]型环加合物。前体通过逆Clasien重排转化为[4 + 2]-型环加合物。因此，乙烯酮将s-顺式1，3-二烯与s-反式二烯区分开来，分别导致最初形成的[4 + 2]和[2 + 2]环加合物。[2 + 2]环加合物通过逆克莱森重排转化为最终的[4 + 2]产物。实验结果与理论计算结果吻合较好。我们发现乙烯酮是一种亲双烯体，不是已知的[2 + 2]环加成，而是通过其C=O键进行[4 + 2]（Diels-Alder）反应。因此，我们在乙烯酮化学方面进行了创新。少

Non-Recognition for 1,3-dienes in ketene reactions has long been an important problem in organic chemistry. We have solved the problem in this research. The mechanism of ketene-diene cycloadditions has been studied both theoretically and experimentally. The paths of the cycloadditions of diphenylketene to cyclic and linear dienes are traced by precise experiments and ab initio calculations. The obtained results show that the addition paths and final products depend upon the structures of dienes, cyclic (with s-cisform) or linear (with s-cis and s-trans forms). The cycloadditions of diphenylketene to cyclic dienes are two-step processes, composed of [4 + 2] addition and Clasien rearrangement, leading to [2 + 2] cycloadducts. On the other hand, those of diphenylketene to linear dienes are two-step processes, composed of [2 + 2] addition and retro-Claisen rearrangement, [4 + 2] cycloadducts.Careful experiments in the reactions of diphenylketene with cyclic (s-cis) 1,3-dienes lead to the f … More irst detection of the Diels-Alder cycloadducts (α-methylenedihydropyrans) by low-temperature NMR spectroscopy. The initially formed cycloadducts are converted by the final [2 + 2]-type products, cyclobutanones, by a [3,3] sigmatropic (Claisen) rearrangement. In contrast, the ketene reacts with open-chain 1,3-dienes (with s-cis and s-trans forms) to form initially both the [2 + 2]-type and [4 + 2]-type cycloadducts. The formers are converted to [4 + 2]-type cycloadducts by retro-Clasien rearrangement. Thus, ketene distinguishes s-cis 1,3-dienes from s-trans ones leading to the initially formed [4 + 2] and [2 + 2] cycloadducts, respectively. The [2 + 2] cycloadduct is converted to the final [4 + 2] product through retro-Claisen rearrangement. Those experimental results are consistent with theoretical ones perfectly. We discovered that ketene is a dienophile not well-known [2 + 2] cycloaddition but for [4 + 2] (Diels-Alder) reactions across its C=O bond. Thus, we have made an innovation in ketene chemistry. Less